The constructor takes a $node_key and a $node_value. The key is used by Tree::Binary::Search to order the nodes in the tree. Both arguments are required, and exception is thrown if they are not present.

Right now, there is nothing preventing you from using anything you want as a node key as long as you create the proper comparison function in Tree::Binary::Search. I would like to test this more, and possibly write some code to faciliate it.